As a method of forming a predetermined thin film on a surface of a substrate to be processed such as glass or silicon wafer, there is known a sputtering method. The sputtering method is an art in which the ions in a plasma atmosphere are accelerated and collided onto targets which are formed into a predetermined shape depending on the composition of the thin film to be formed on a surface of a substrate, and in which the sputtered particles (atoms of the targets) are scattered for getting adhered and deposited on the surface of the substrate to thereby form a predetermined thin film. Recently the method is used, in the process of manufacturing a flat panel display (FPD), to form a thin film such as an ITO and the like on a large-area substrate.
The following is known as a sputtering apparatus in which a thin film can be efficiently formed at a constant film thickness on a substrate of a larger area. In other words, this sputtering apparatus has: a plurality of targets of the same shape which are disposed at an equal distance from one another, opposite to the substrate to be processed in a vacuum chamber; and an AC power supply which charges a predetermined potential to those respective targets which make pairs, out of the disposed targets, at a predetermined frequency while alternately changing the polarity (by reversing the polarity). While introducing a predetermined sputtering gas in a vacuum, electric power is applied through the AC power supply to the targets that make pairs. By alternately switching each of the targets between anode electrode and cathode electrode, glow discharge is caused to be generated between the anode electrode and the cathode electrode. Plasma atmosphere is thus formed to thereby sputter each of the targets (see, e.g., patent document 1).
Suppose that AC voltage of substantially sinusoidal wave is charged to each of the targets by using the above-mentioned AC power supply. Then, since the output voltage and the output current are continuously varying, it is difficult to detect an anomalous electric discharge (arcing) when it occurs for some reason or other, e.g., during the above-mentioned glow discharge and to take the necessary steps such as to cut off the output to the target depending on the result of detection.
As a solution, it is known to dispose a rectifying circuit which supplies DC power, and a bridge circuit which is connected to the positive and the negative output ends of the rectifying circuit and is made up of four switching elements so that AC pulsed power potential is charged to the paired targets alternately at a predetermined frequency (see, e.g., patent document 2).
In the above-mentioned art, however, since discharge is caused to take place between the targets that make a pair, the discharge current flows only between the targets. In such a case, if the output frequency is lowered (e.g., 10 kHz or less) in order to detect the anomalous electric discharge or to facilitate the subsequent processing work, plasma tends to be partially generated only in front of the targets that are being outputted (or that are receiving outputs. As a result, there is a problem in that it becomes difficult to attain a uniformity in the film thickness distribution of the thin film to be formed on the surface of the substrate.
Patent Document 1: JP-A-2005-290550
Patent Document 2: JP3639605